Investigate compatiblity/interoperability with legacy containers

[m1cr0man]

The containers option set + modules are in heavy use by people already. RFC108 will supersede them but it is important to have a migration path. Before NixOS/nixpkgs#216025 can be merged safely this needs to be investigated.

[eldritch-cookie]

what exactly would completing this issue look like? do we need a tool to automatically upgrade from old containers to new, or would just having a manual with the needed steps be enough?

[m1cr0man]

For this particular issue, I'm curious to test the following interactions:

• NixOS system with running legacy container, then adding an imperative container to it.
• NixOS system with running legacy container plus a declarative (rfc108) container

Maybe the answer is "legacy containers don't work on networkd" which would be an acceptable answer to just close the ticket. I suspect it won't be quite that simple, and I would like to (if possible) make legacy and rfc108 containers play nicely together to ease migration for users.

Wrt upgrading, this was discussed previously in the original PR, and documenting the process for performing such an upgrade will be encapsulated by #5